Finally a nice day, and some spare time so I could mount the thrusters to the boat. I used some stainless steel M6 threaded rod and some DIN rail. DIN Rails is great. It's cheap, has mounting holes and it is very stiff due to the shape.
I kept plenty of length on the M6 rods so I can adjust the depth of the thrusters later, if required.
Once all nuts are tightened the whole construction became quite rigid so it seems like this is going to work. The only thing that worries me is that the metal is sticking out quite a bit to the side and that it will definitely collect weed and algae.